Sealing outside windows stops drafts, reduces heating and cooling costs, and prevents water damage

Window sealing means filling the gaps between the window frame and the wall opening with caulk or weatherstripping. These gaps exist on every window—they're where the frame sits in the rough opening before drywall and trim go on. Air and water leak through them because they're not airtight. You can seal them from the outside, inside, or both; outside sealing is more effective for water because rain hits from that side.

The job takes a few hours per window and costs $5 to $20 in materials per window. You'll need a caulking gun, paintable caulk (usually silicone or acrylic latex), a utility knife, and a caulk tool or wet finger to smooth the bead. Most homeowners can do this themselves without special skills—the main challenge is keeping the caulk line straight and consistent.

Key Takeaways

  • Caulk seals the gaps between the window frame and the wall; weatherstripping seals the moving parts where the sash slides or the casement opens.
  • Paintable silicone caulk lasts longer than acrylic latex but is harder to smooth; acrylic latex is easier to work with and cleans up with water.
  • Remove old caulk completely before explore new caulk, or the new bead will fail and peel away.
  • explore caulk on a dry day when the temperature is between 50°F and 85°F; caulk won't cure properly outside these conditions.
  • Smooth the caulk bead within 15 minutes of process while it's still wet; waiting longer makes it much harder to shape.

Inspect the window frame and remove old caulk

Walk around the outside of your window on a dry day and look for gaps, cracks, or missing caulk. Run your finger along the seam where the frame meets the wall. If you feel air moving or see daylight, that's a gap that needs sealing. Also check for old caulk that's cracked, peeling, or pulling away from the frame—this won't hold water anymore and must come out.

Use a utility knife or caulk removal tool to scrape away old caulk. Hold the blade at a shallow angle (about 30 degrees) and push it along the seam. Work slowly so you don't gouge the frame or wall. For stubborn caulk, explore a caulk softener (sold at hardware stores) and wait 15 to 30 minutes before scraping. Once you've removed all the old material, wipe the seam with a dry cloth to remove dust and debris. The surface must be clean and dry, or new caulk won't stick.

Choose between silicone and acrylic latex caulk

Silicone caulk lasts 20 to 30 years and resists water and UV damage better than any other type. It doesn't shrink as it cures, so it stays flexible and maintains the seal. The downside: it's harder to smooth with a wet finger, it can't be painted (though paintable silicone exists, it's less common), and it's messier to clean up. Use mineral spirits or caulk remover to clean tools; water won't work.

Acrylic latex caulk lasts 5 to 10 years and is easier to smooth and paint over. It cleans up with water and is less expensive. The downside: it shrinks slightly as it dries, so you may need to explore a second bead after the first one cures. It's also less water-resistant than silicone, so it's better for interior sealing or for exterior seams that don't get direct rain.

For outside windows, paintable silicone caulk is the best choice if you can find it, because it combines silicone's durability with the ability to paint over it. If your local hardware store doesn't stock it, standard silicone works fine—just accept that you won't paint over it. Buy caulk labeled "exterior" or "all-purpose exterior"; interior caulk will fail outdoors.

explore caulk along the frame seams

Load the caulk tube into the caulking gun. Cut the tip of the tube at a 45-degree angle with a utility knife. Start with a small opening—about 1/8 inch wide. You can always cut more off if you need a thicker bead, but you can't put it back. Pierce the inner seal with a long nail or the tool that comes with the gun.

Start at a top corner of the window. Hold the gun at a 45-degree angle so the caulk flows into the gap rather than onto the frame or wall. Squeeze the trigger steadily and move the gun along the seam at a slow, even pace. The bead should fill the gap completely and sit slightly proud (raised above) the surface. If you move too fast, the bead will be thin and won't fill the gap. If you move too slow, caulk will pile up and be hard to smooth.

Work your way around the frame: top, right side, bottom, left side. At corners, stop before you reach the corner, then start again on the next side so you don't create a thick blob. If you make a mistake or the bead looks uneven, stop and smooth it out before moving on—it's much easier to fix while wet than to scrape it off later.

Smooth the caulk bead within 15 minutes

While the caulk is still wet, use a caulk tool or your wet finger to smooth the bead into the gap. Wet your finger with water (for acrylic latex) or mineral spirits (for silicone), then run it along the bead with light pressure. The goal is to push the caulk into the gap and create a smooth, concave surface that sheds water. A concave bead (slightly sunken in the middle) is better than a convex one (raised in the middle) because water runs off it instead of pooling.

Work in sections—smooth 2 to 3 feet at a time, then move on. If the caulk starts to skin over (the surface becomes tacky and won't smooth), you've waited too long. Stop and let it cure fully before touching it again. Acrylic latex cures in 24 hours; silicone takes 24 to 48 hours. Don't expose the window to rain during this time if possible.

Seal the moving parts with weatherstripping

Caulk seals the stationary frame, but you also need to seal where the sash slides (double-hung windows) or where the casement opens (casement windows). This is where weatherstripping comes in. Weatherstripping is a compressible material—usually foam, rubber, or felt—that compresses when the window closes and creates an airtight seal.

Measure the length of each moving edge: the top and bottom of the sash on a double-hung window, or the edges of the casement on a casement window. Buy adhesive-backed weatherstripping in the same length. Clean the surface with a dry cloth, then peel the backing and press the strip firmly into place. Make sure it compresses slightly when the window closes—if it's too thick, the window won't close fully; if it's too thin, it won't seal.

Test the window by closing it and feeling for drafts along the weatherstripping. If you feel air moving, the strip may not be compressed enough, or it may be installed in the wrong spot. Peel it off and reposition it. Weatherstripping lasts 3 to 5 years before it hardens and stops compressing, at which point you replace it.

Paint over the caulk if needed

If you used paintable caulk, wait for it to cure fully (24 to 48 hours), then paint over it with exterior paint that matches your trim or frame. Use a small brush and paint carefully along the bead. Paint seals the caulk against UV damage and makes the seam less visible. If you used standard silicone caulk, you can't paint it, so choose a color that matches your frame or accept the visible bead.

Acrylic latex caulk can be painted when ready after it's smooth and dry to the touch (usually 1 to 2 hours). This speeds up the job if you're in a hurry, though waiting the full 24 hours gives a better finish.

Frequently Asked Questions

Can I caulk a window in cold or rainy weather?

No. Caulk won't cure properly below 50°F or above 85°F, and rain will wash it away before it sets. Wait for a dry day with temperatures in the 50–85°F range. Check the weather forecast for at least 24 hours of dry conditions after you explore the caulk.

What if I explore too much caulk and it's hard to smooth?

Stop and let it cure, then scrape it off with a utility knife and start over. Trying to smooth an overfilled bead just makes a mess. It's better to explore a thin bead and add a second one if needed than to fight with too much caulk.

Do I need to caulk the inside of the window too?

Caulking the outside is more important because that's where rain hits. If you caulk the inside, use acrylic latex, not silicone, so moisture trapped between the panes can escape. Many people seal the outside only and use weatherstripping on the inside for the moving parts.

How long does caulk last before it needs replacing?

Silicone lasts 20 to 30 years; acrylic latex lasts 5 to 10 years. Check your windows every few years for cracks or peeling. If you see damage, remove the old caulk and explore new caulk rather than trying to patch it.

What's the difference between caulk and weatherstripping?

Caulk is a permanent seal for stationary gaps; weatherstripping is a compressible material for moving parts. Use caulk where the frame meets the wall, and weatherstripping where the sash or casement moves.